Insertion compounds can be defined as those compounds wherein an amount of an element, molecule, or other species can be inserted into the host structure of the compound and then removed again without having irreversibly altered the host structure. Thus, while the host structure may be altered by insertion of a species, the original structure is retained upon subsequent removal of the species. Generally, only minor alterations of the host structure can occur before insertion is no longer reversible. The insertion or extraction of the species beyond the reversible range often results in the compound undergoing a phase transition to become another insertion compound with a different host structure. Often the phase transition itself is reversible.
Insertion compounds have proven useful for a variety of applications such as use as ion exchangers, but they are particularly suitable for use in non-aqueous rechargeable batteries. The excellent reversibility of some of these compounds upon insertion with lithium makes such compounds very attractive for use as electrodes in lithium rechargeable batteries. Several manufacturers, including Sony Energy Tec. and AT Battery, have made non-aqueous lithium-ion type batteries commercially available wherein both the cathode and anode electrodes are lithium insertion compounds. In each case, the cathode is a lithium cobalt oxide compound and the anode is a carbonaceous material.
These commercial lithium-ion type batteries are constructed using components that may be somewhat sensitive to water vapour but are otherwise stable in air. Thus, the batteries can be assembled economically under dry air conditions at the worst. It is therefore important to choose electrode materials that are air stable. Lithiated carbonaceous material anodes are not stable in air, so batteries are usually made in a completely discharged state wherein all the lithium in the battery resides in the cathode. Preferable cathode materials therefore have the maximum possible amount of lithium inserted while still being air stable. Additionally, cathode materials preferably are chosen that allow the maximum possible amount of lithium to be reversibly removed and re-inserted, hence providing the maximum battery capacity.
Many lithium transition metal oxide compounds may be used as cathodes in lithium-ion battery products. Along with LiCoO.sub.2 (used in the Sony Energy Tec. product and described in U.S. Pat. No. 4,302,518 of Goodenough), other possible compounds include LiNiO.sub.2, (also described in the aforementioned U.S. Patent), LiMn.sub.2 O.sub.4 (described in U.S. Pat. No. 4,507,371), and other lithium manganese oxide compounds. Since cobalt is relatively rare, LiCoO.sub.2 is relatively expensive compared to the latter two compounds. Both Co and Ni containing compounds are considered to be potential cancer causing agents and are therefore subject to strict handling requirements, particularly with respect to airborne particulate levels. Lithium manganese oxides are less of a toxicity concern and are relatively inexpensive. For these reasons, such oxides would be preferred in commercial lithium-ion type batteries if other performance requirements can be maintained.
To enhance the operating capacity of lithium-ion type batteries, it has been considered desirable, where possible, to insert additional lithium into the cathode material using chemical means prior to battery construction. For example, LiMn.sub.2 O.sub.4 with the spinel structure can be further lithiated reversibly up to a stoichiometry of Li.sub.2 Mn.sub.2 O.sub.4 using a reaction involving LiI as described in U.S. Pat. No. 5,196,279. However, iodine compounds can be quite corrosive and this creates potential problems when contemplating such a process for large scale manufacturing. Additionally, while this Li.sub.2 Mn.sub.2 O.sub.4 compound is relatively stable under ambient conditions (ie. no significant degradation noticed over several days), it is not completely stable in air.
In general, lithium transition metal oxides are not stable in air. Only if the lithium atoms are sufficiently tightly bound to the host will they not react with water vapour, oxygen, or CO.sub.2 in the air. A direct measure of the binding energy of the lithium atoms in a lithium transition metal oxide is the voltage of said oxide with respect to lithium metal in a non-aqueous battery. Empirically, it has been determined in J. R. Dahn et al, J. Electrochem Soc., 138, 2207 (1991) that lithium insertion compounds are effectively air stable if the voltages of said compounds versus lithium are greater than 3.3 .+-.0.2 V. As shown in J. M. Tarascon et al., J. Electrochem. Soc., Vol. 138, No. 10 (1991), the aforementioned Li.sub.2 Mn.sub.2 O.sub.4 compound has a voltage versus lithium of 2.97 V and thus reacts eventually with the moisture in the air to form LiOH and LiMn.sub.2 O.sub.4. Thus, while it is possible to construct a lithium-ion battery in air using this compound, special handling and storage procedures are still required to minimize the accumulated reaction with air to an acceptable level in practice. Generally, it would be expected that direct exposure to an aqueous environment would result in a more serious degradation of this compound. Thus, for example, the use of Li.sub.2 Mn.sub.2 O.sub.4 in an aqueous battery application seems impractical.
Insertion compounds are used as the major active ingredient in both electrodes of typical lithium ion batteries. However, insertion compounds can also be used as an additive in such batteries. U.S. Pat. No. 5,278,000 and Japanese laid-open patent application number 04-022066 both describe the use of insertion compounds as cathode additives in lithium ion batteries. The former describes the use of a conventional form of Li.sub.2 Mn.sub.2 O.sub.4 to prevent overdischarge, while the latter describes the use of Li.sub.2 CuO.sub.2 to improve performance on overdischarge.